Thick-film copper conductor inks

ABSTRACT

Improved copper conductor inks useful in fabricating multilevel circuits are provided. The inks comprise copper powder, a devitrifying glass frit which does not begin to flow until the furnace temperature is above about 700° C., and a suitable organic vehicle. Devitrifying glass frits with these properties include a zinc-calcium-aluminum-silicate glass frit, a zinc-magnesium-barium-aluminum-silicate glass frit, a zinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass frit and mixtures thereof. The inks are advantageous in that they form copper conductor layers having excellent properties without the inclusion of traditional flux materials such as bismuth oxide.

This application is a continuation-in-part of U.S. patent applicationSer. No. 914,303, filed Oct. 2, 1986 now abandoned.

This invention relates to thick-film copper conductor inks and their usein fabricating multilayer electrical circuit structures.

BACKGROUND OF THE INVENTION

The use of specialized ink formulations to form thick films havingvarious functions on suitable substrates in the construction ofmultilayer integrated circuits is known in the art. Such technology isof increasing interest in the production of very dense multilayercircuit patterns on various substrates for a wide variety ofapplications in the electronics industry.

Thick-film multilayer structures based on copper conductors typicallyare comprised of at least two patterned layers of copper conductorseparated by a dielectric layer. The patterned conductor layers areconnected by copper deposited in vias in the dielectric layer. Suchstructures are formed by multiple deposition and firing of layers ofcopper and dielectric inks.

Such multilayer circuit structures utilizing copper as the conductormetal have a number of problems. The most common is failure caused bythe development of electrical shorts due to interactions between fluxmaterials of the copper conductor ink and the dielectric layer whichtake place during the multiple firings necessary to fabricate amultilayer integrated circuit. The responsible materials present inconventional copper conductor inks include copper oxide which forms uponexposure of the ink to air or an oxidizing atmosphere and flux materialssuch as lead oxide and bismuth oxide. These materials will penetrate aporous dielectric material, particularly if it contains large modifierions such as lead, barium and bismuth. The penetration of such materialsis enhanced by the fact that the multiple firing steps areconventionally carried out at temperatures well above the temperature atwhich copper oxide forms an eutectic mixture with lead or bismuthoxides.

A second problem common to both dielectric and copper conductor inks isthe entrapment of gaseous materials formed during repeated firings as aresult of incomplete removal of the carbonaceous residue of the organicvehicle present in conventional ink formulations. The escaping gaseousmaterial can cause blistering and peeling of the thick films formed fromthe inks during subsequent firings and is also responsible for theporosity of dielectric films.

An approach to reducing the incidence of electrical shorts from theseproblems is to formulate dielectric inks which form thick films havingreduced porosity.

A second approach is to treat functional inks, e.g. dielectric andcopper conductor inks, with an oxidizing or reducing plasma prior tofiring as disclosed by Prabhu et al. in U.S. Pat. No. 4,619,836, issuedOct. 28, 1986. The plasma treatment removes the carbonaceous residue ofthe organic vehicle present in conventional ink formulations.

A third possibility is the formulation of improved copper conductorinks. Such improved inks are provided in accordance with this invention.

SUMMARY OF THE INVENTION

The improved copper conductor inks of the present invention comprisecopper powder, an organic vehicle and a devitrifying glass frit whichdoes not begin to flow until the furnace temperature is above about 700°C. Devitrifying glass frits having these properties include azinc-calcium-aluminum-silicate glass frit, asinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass frit, azinc-magnesium-barium-aluminum-silicate glass frit and mixtures thereof.The subject inks are formulated without bismuth oxide, thereby virtuallyeliminating the formation of eutectic phases with copper oxide whichreadily penetrate into dielectric materials. The subject inks areutilized in the fabrication of multilayer integrated circuit structures.

DETAILED DESCRIPTION OF THE INVENTION

The copper powder utilized in the subject conductive inks is pure copperhaving a particle size of from about 1 to 5 micrometers. The coppercomprises from about 65 to about 85, preferably from about 75 to about80, percent by weight of the subject ink compositions.

The devitrifying glass frits utilized in the conductive inks of thepresent invention do not begin to flow until the furnace temperature isabove about 700° C. Suitable devitrifying glass frits having theseproperties include a zinc-calcium-aluminum-silicate glass frit, azinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass frit, azinc-magnesium-barium-aluminum-silicate glass frit and mixtures thereof.The devitrifying glass frits may be utilized individually or combined inany proportion and comprise from about 5 to about 15, preferably fromabout 6 to about 9, percent by weight of the subject inks.

The zinc-calcium-aluminum-silicate glass frit of the novel inks of thisinvention is disclosed in a dielectric ink in copending Hang et al. U.S.patent application Ser. No. 914,301, entitled "DIELECTRIC INKS FORMULTILAYER COPPER CIRCUITS", filed on Oct. 2, 1986, and comprises, on aweight basis:

(a) from about 7 to 12, preferably from about 8 to about 10, percent ofzinc oxide;

(b) from about 25 to 45, preferably from about 29 to about 38, percentof calcium oxide;

(c) from about 10 to 20, preferably from about 11 to about 18.5, percentof aluminum oxide;

(d) from about 35 to 50, preferably from about 37 to about 44, percentof silicon dioxide;

(e) from 0 to about 2, preferably from about 0.5 to about 1, percent ofphosphorus pentoxide; and

(f) from 0 to about 5, preferably from about 2 to about 3, percent ofzirconium silicate.

The zinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass fritof the inks of this invention is disclosed in a dielectric ink incopending U.S. patent application Ser. No. 914,302 of Hang et al.entitled "DIELECTRIC INKS FOR MULTILAYER COPPER CIRCUITS", filed on Oct.2, 1986. This frit comprises, on a weight basis:

(a) from about 15 to about 25, preferably from about 16 to about 22,percent of zinc oxide;

(b) from about 10 to about 25, preferably from about 16 to about 22,percent of magnesium oxide;

(c) from about 3 to about 12, preferably from about 5 to 10, percent ofbarium oxide;

(d) from about 5 to about 20, preferably from about 8 to about 11,percent of aluminum oxide;

(e) from about 35 to about 50, preferably from about 39 to about 43,percent of silicon dioxide;

(f) from about 0.5 to about 3, preferably from about 1 to about 2,percent of phosphorus pentoxide; and

(g) from about 1 to about 5, preferably from about 2 to about 3, percentof zirconium silicate.

In order to have a wider range of control of the crystallization rate ofthe zinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass frit,the lower weight percentage limits of both phosphorus pentoxide andzirconium silicate may be lowered to include zero. Thus, the inks ofthis invention also include a devitrifyingzinc-magnesium-barium-aluminum-silicate glass frit. This frit comprises,on a weight basis:

(a) from about 15 to about 25, preferably from about 16 to about 22,percent of zinc oxide;

(b) from about 10 to about 25, preferably from about 16 to about 22,percent magnesium oxide;

(c) from about 3 to about 12, preferably from about 5 to about 10,precent of barium oxide;

(d) from about 5 to about 20, preferably from about 8 to about 11,percent of aluminum oxide;

(e) from about 35 to about 50, preferably from about 39 to about 43,percent of silicon dioxide;

(f) from 0 to about 3 percent of phosphorus pentoxide; and

(g) from 0 to about 5 percent of zirconium silicate.

When it is desirable to moderate the flow of the copper ink duringfiring, by increasing the crystallization rate of the glass frit, from 0to about 0.5 weight percent of phosphorus pentoxide and from 0 to about1 weight percent of zirconium silicate should be used.

The devitrifying glass frits of the subject copper conductor inks areparticularly advantageous in that they have a very high softeningtemperature, i.e. they do not begin to flow until the furnacetemperature is above 700° C. Since the dried ink layers remain permeableto the passage of gaseous materials until they begin to significantlyflow and densify, the subject frits provide substantial extra time inthe furnace where furnace gases may penetrate the frit and remove thelast carbonaceous residues of the binder. Because the high softeningtemperature of the glass frit provides efficient removal of carbonaceousresidues from the dried ink, it is not necessary to treat the subjectinks before firing in an oxidizing or reducing plasma as described inthe above-mentioned Prabhu et al. U.S. Pat. No. 4,619,836.

A second and unexpected advantage of the devitrifying glass frits of thesubject inks is that they can be formulated into a copper conductor inkhaving excellent properties without the inclusion of traditional fluxmaterials such as lead oxide and, particularly, bismuth oxide.

A further advantage of the zinc-calcium-aluminum-silicate glass frit ofthis invention is that it has an expansion coefficient close to that ofconventional alumina circuit boards in both the vitreous and devitrifiedstates. This is an important departure from the glass frits contained inmost conventional inks which have a marked difference in expansionbetween their vitreous and devitrified states. This is an advantagesince this frit is slow to crystallize.

The zinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass fritof the subject inks does not possess a similar coefficient of expansionin the vitreous and devitrified states. However, since it completes itscrystallization very rapidly, i.e. within a single firing cycle, theeffect is the same, i.e. neither glass frit undergoes significantchanges in expansion from layer to layer during multiple firings.

The organic vehicles are solutions of resin binders such as, forexample, cellulose derivatives, particularly ethyl cellulose, syntheticresins such as polyacrylates, polymethacrylates, polyesters, polyolefinsand the like in a suitable solvent. A preferred binder ispoly(isobutylmethacrylate). In general, conventional solvents utilizedin inks of the type described herein may be used. Preferred commerciallyavailable solvents include, for example, pine oil, terpineol, butylcarbitol acetate, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate,available from Texas Eastman Company under the trademark Texanol and thelike. The vehicles suitably contain from about 5 to about 25 percent byweight of the resin binder. However, it may be necessary to add moresolvent to the organic vehicle to adjust the ink rheology. Thus, theorganic vehicle may contain from about 2 to about 25 percent by weightof the resin binder.

The above resin binders may be utilized individually or in anycombination of two or more. A suitable viscosity modifier can be addedto the resin material if desired. Such a modifier can be, for example, acastor oil derivative available from N.L. Industries under the trademarkThixatrol.

The vehicles of the subject inks may also contain up to about 25,preferably from about 10 to about 20, percent by weight, based on thevehicle, of a suitable wetting agent of the type conventionally used incopper conductor inks to aid in coating the particles of copper powderwith the organic vehicle. As is the case with all components of theorganic vehicle, the wetting agent must fire cleanly in nitrogen, i.e.,without leaving a carbonaceous residue. A preferred wetting agent is adispersion of a complex multifunctional, aliphatic hydrocarbon in analiphatic hydrocarbon oil available under the trademark Hypothiolate 100from Central Compounding Company, Chicago, Illinois. The vehicles alsosuitably contain from about 0.5 to about 10, preferably from about 1 toabout 3, percent by weight of a surfactant such as oleylamine, availableas Armeen O, or a high molecular weight N-alkyl-1,3-diaminopropanedioleate, available as Duomeen TDO, both from AKZO Chemie America. Theorganic vehicle comprises from about 5 to about 25 percent preferablyfrom about 12 to about 16 percent, by weight, or the subject inks.Regardless of the vehicle utilized, it is important that the homogeneityof the ink be maximized. Therefore, mixing is suitably carried out in aconventional apparatus which mixes in combination with subjecting thedispersion to high shearing action.

The improved copper conductor inks of this invention are applied to thesubstrate structures by conventional means, e.g. screen printing,brushing, spraying and the like, with screen printing being preferred.The coating of ink is dried in air at 100°-125° C. for about 15 minutes.The resulting film is then fired in nitrogen at 850° to 950° C. for from4 to 10 minutes to form copper conductors comprising from about 70 toabout 95 percent by weight of the copper powder and from about 5 toabout 30 percent by weight of the glass frit. The subject inks are mostsuitably utilized for buried layers of copper conductor in a multilayercircuit in view of their compatibility with conventional boards and theimproved dielectric inks described herein. It is preferred in accordancewith this invention that a precoating of dielectric material bedeposited over a substrate, e.g. a conventional alumina substrate,before the initial layer of the subject copper conductor is printedthereon.

Although the subject conductor inks adhere well to conventional aluminacircuit boards, the use of a dielectric precoat is considered beneficialin that it enhances adherance of the subject conductor and virtuallyeliminates the possibility of separation from the substrate duringsubsequent firings. In general, a thin coating of dielectric material,i.e. from about 10 to 20 micrometers, is contemplated herein. Suitably,such a coating is uniformly deposited on the substrate as an initialstep in the formulation of multilayer circuits. Although the choice of adielectric material is not particularly critical, other than arequirement that it have an expansion coefficient close to that of thecircuit board, it is preferred that a dielectric ink be utilized that isbased on the glass frits of the subject copper conductor ink. Such inksare disclosed in the aforementioned Hang et al. U.S. patent applicationSer. Nos. 914,301 and 914,302. Such dielectric inks generally comprisefrom about 50 to about 75 percent by weight of the glass frit, up toabout 30 percent by weight of a suitable ceramic filler and from about15 to about 30 percent by weight of a suitable organic vehicle. Suitableceramic fillers include alumina powder (Al₂ O₃), barium dimagnesiumdisilicate (BaMg₂ Si₂ O₇), dimagnesium borate (Mg₂ B₂ O₅), zirconiumsilicate (ZrSiO₄), dimagnesia silicate (2MgO-SiO₂), dimagnesia dialuminapentasilicate (2MgO-2Al₂ O₃ -5SiO₂) and mixtures thereof.

Copper conductor layers formed from the inks of this invention areadvantageous in that they demonstrate good conductivity and resistanceto oxidation. In addition, copper conductor layers formed from the inksof this invention possess excellent compatibility with improveddielectric materials such as those described in the aforementioned Hanget al. applications.

The following Examples further illustrate this invention, it beingunderstood that the invention is in no way intended to be limited to thedetails described therein. In the Examples, all parts and percentagesare on a weight basis and all temperatures are in degrees Celsius,unless otherwise stated.

EXAMPLE 1

A copper conductor ink was prepared by mixing 76.9 parts of copperpowder having an average particle size of 3 micrometers and 7.7 parts ofa devitrifying glass frit comprising: 9.09 percent of zinc oxide, 30.40percent of calcium oxide, 18.28 percent of aluminum oxide and 42.23percent of silicon dioxide. The solid ingredients were mixed with 15.4parts of an organic vehicle consisting of a 6 percent solution of ethylcellulose in Texanol, additionally containing the wetting agentsHypothiolate 100 and Armeen O in concentrations of 17 percent and 3percent, respectively, based on the vehicle. The ink was initiallyhand-mized and then mixed on a three-roll mill to obtain a pastesuitable for screen printing. Additional solvent was added as necessaryto assure proper rheology.

A conventional alumina board was initially coated with a 15 micrometerthick layer of dielectric material by printing and firing a dielectricink. The dielectric ink was comprised of 57.4 parts of theabove-described glass frit, 8.6 parts of alumina having a particle sizeof 3 micrometers, 10.5 parts of barium dimagnesium disilcate having aparticle size of 3-5 micrometers and 23.5 parts of a vehicle comprisinga 20 percent solution of poly(isobutylmethacrylate) in Texanol alsocontaining one percent of Duomeen TDO based on the vehicle. The copperink was printed onto the fired dielectric material, dried in air at 125°for 15 minutes and fired in nitrogen at 900° for 10 minutes to form alayer 12.5 micrometers thick. The sequential printing and firing ofdielectric and copper inks was repeated twice more to produce amultilayer structure having three buried layers of copper conductor. Foreach dielectric isolation layer, three separate printings and firings ofthe dielectric ink were carried out. It was found by biasing pairs ofthe copper conductor layers that no contact had been made through any ofthe dielectric material. Microscopic examination showed no evidence ofblistering or separation of the copper conductor from the dielectricmaterial and substantially no evidence of copper penetration into thedielectric material.

EXAMPLE 2

The procedure of Example 1 was repeated utilizing in the copperconductor a glass frit comprised of 21.81 percent of zinc oxide, 19.25percent of magnesium oxide, 5.88 percent of barium oxide, 9.38 percentof aluminum oxide, 39.68 percent of silicon dioxide, 2.00 percent ofphosphorus pentoxide and 2.00 percent of zirconium silicate.

The dielectric ink was comprised of 67.7 percent of the above glassfrit, 5.8 percent of alumina, 3.9 percent of barium dimagnesiumdisilicate and 22.6 percent of the organic vehicle utilized to preparethe dielectric ink of Example 1.

Biasing a multilayer structure prepared as in Example 1 showed noevidence of blistering or separation of the copper conductor and nodiscernible penetration of components of the copper ink into thedielectric material.

EXAMPLE 3

A conventional alumina board was initially coated with a thin layer ofdielectric material as in Example 1. The copper conductor ink of Example1 was printed thereover to form a series of isolated parallel lines 375micrometers wide separated by spaces of equal width. The copper ink wasdried in air at 125° for 15 minutes and fired in nitrogen at 900° for 10minutes. The dielectric ink of Example 1 was printed, dried and firedthereover. Openings or vias were left in the dielectric layer overlyinga portion of the copper conductor.

A copper via-fill ink was prepared as follows: a devitrifying glass fritconsisting of: 21.8 percent of zinc oxide; 16.5 percent of magnesiumoxide; 7.8 percent of barium oxide; 39.2 percent of silicon dioxide;10.7 percent of aluminum oxide; 1.0 percent of phosphorus pentoxide; and3.0 percent of zirconium silicate and a vitreous glass frit consistingof: 51.59 percent of barium oxide, 12.58 percent of calcium oxide; 15.62percent of boron trioxide; and 20.21 percent of silicon dioxide wereseparately prepared and reduced to a particle size of about threemicrometers. Solid ingredients consisting of 65 percent of copper powderhaving an average particle size of three micrometers, 14 percent of thedevitrifying glass frit and 4 percent of the vitreous glass frit werethoroughly mixed by hand.

The solid ingredients were mixed with 17 parts of an organic vehicleconsisting of a 20 percent solution of poly(isobutylmethacrylate) inTexanol additionally containing, as wetting agents, 17 percent ofHypothiolate, 100 and 3 percent of Armeen O. The ink was initiallyhand-mixed and then mixed on a three-roll mill to obtain a pastesuitable for screen printing. Additional solvent was added as necessaryto assure proper rheology. The via-fill ink is described in copendingU.S. patent application Ser. No. 914,296 of Prabhu et al. entitled"THICK-FILM COPPER VIA-FILL INKS", filed Oct. 2, 1986.

The via-fill ink was printed into the spaces in the dielectric ink,dried in air at 125° for 15 minutes and fired in nitrogen at 900° for 10minutes. The thickness of the dielectric/copper via-fill was 15micrometers. The dielectric/copper via-fill depositions were repeatedthree times to form a final thickness of 45 micrometers. A layer of thecopper conductor ink of Example 1 was deposited and fired over thestructure so that a portion was in contact with the copper via-fill.This procedure was repeated three times to obtain a multilayercopper-based device having four buried layers. In total, 25 firings wererequired to complete the multilayer circuit.

The structure was biased through electrical contacts made to the copperlayers. No evidence of shorting or loss of contact was observed in anyof the copper layers.

EXAMPLE 4

Example 3 was repeated utilizing the copper conductor ink and dielectricink described in Example 2. Biasing of the resulting multilayer circuitstructure produced results comparable to those obtained in Example 3.

We claim:
 1. A multilayer, copper-based integrated circuit structurecomprising a circuit board having thereon at least two patterned layersof copper conductor separated by a dielectric layer having vias filledwith copper contacting said copper conductor layers, said copperconductors comprising from about 70 to about 95 weight percent copperpowder and from about 5 to about 30 weight percent glass frit selectedfrom the group consisting of zinc-calcium-aluminum silicate glasses andzinc-magnesium-barium-aluminum-zirconium-phosphosilicate glasses whichdo not flow at temperatures below about 700° C.
 2. A multilayer,copper-based integrated circuit structure according to claim 1comprising a suitable circuit board having thereon at least twopatterned layers of a copper conductor, said layers being separated by adielectric layer having vias therein, said vias being filled with copperto contact said conductor layers, the copper conductors comprising on aweight basis,(a) from about 70 to about 95 percent of copper powder; and(b) from about 5 to about 30 percent of a devitrifying glass fritcomprising on a weight basis: (c) from about 15 to about 25 percent ofzinc oxide; (d) from about 10 to about 25 percent of magnesium oxide;(e) from about 3 to about 12 percent of barium oxide; (f) from about 5to about 20 percent of aluminum oxide; (g) from about 35 to about 50percent of silicon dioxide; (h) from 0 to about 3 percent of phosphoruspentoxide; and (i) from 0 to about 5 percent of zirconium silicate.
 3. Acircuit structure in accordance with claim 1, wherein the devitrifyingglass frit is a zinc-magnesium-barium-aluminum-silicate glasscomprising, on a weight basis:(a) from about 15 to about 25 percent ofzinc oxide; (b) from about 10 to about 25 percent of magnesium oxide;(c) from about 3 to about 12 percent of barium oxide; (d) from about 5to about 20 percent of aluminum oxide; (e) from about 35 to about 50percent of silicon dioxide; (f) from 0 to about 0.5 percent ofphosphorus pentoxide; and (g) from 0 to about 1 percent of zirconiumsilicate.
 4. In a multilayer, copper-based integrated circuit structurecomprising a suitable circuit board having th thereon at least twopatterned layers of a copper conductor, said layers being separated by adielectric layer having vias therein, said vias being filled with copperto contact said conductor layers, the improvement wherein the copperconductors comprise on a weight basis:(a) from about 70 to about 95percent of copper powder; and (b) from about 5 to about 30 percent of adevitrifying glass frit selected from the group consisting of azinc-calcium-aluminum-silicate glass frit, azinc-magnesium-barium-aluminum-zirconium-phosphosilicate glass frit andmixtures thereof.
 5. A circuit structure in accordance with claim 1,wherein the devitrifying glass frit is a zinc-calcium-aluminum-silicateglass frit comprising, on a weight basis:(a) from about 7 to about 12percent of zinc oxide; (b) from about 25 to about 45 percent of calciumoxide; (c) from about 10 to about 20 percent of aluminum oxide; (d) fromabout 35 to about 50 percent of silicon dioxide; (e) from 0 to about 2percent of phosphorus pentoxide; and (f) from 0 to about 5 percent ofzirconium silicate.
 6. A circuit structure in accordance with claim 1wherein the devitrifying glass frit is azinc-magnesium-barium-aluminum-zirconium-phosphosilicate glasscomprising, on a weight basis:(a) from about 15 to about 25 percent ofzinc oxide; (b) from about 10 to about 25 percent of magnesium oxide;(c) from about 3 to about 12 percent of barium oxide; (d) from about 5to about 20 percent of aluminum oxide; (e) from about 35 to about 50percent of silicon dioxide; (f) from about 0.5 to about 3 percent ofphosphorus pentoxide; and (g) from about 1 to about 5 percent ofzirconium silicate.